The Festive Side of Carbon Dioxide

Scientists Uncork Champagne's Complexity and the Wondrous Micro-Mechanics of Bubbles

By

Robert Lee Hotz

Updated Dec. 18, 2009 12:01 a.m. ET

There are 20 million bubbles in a bottle of champagne and every one of them alters the taste, scent and fluid dynamics of the sparkling wine, say researchers studying the chemistry of carbonation and the physics of fizz.

During centuries of artisanal trial and error, winemakers had learned surprisingly little about how a sparkling wine's most active ingredient affected its chemistry of aroma and flavor. To understand the essence of its effervescence, the researchers analyzed champagne bubbles with mass spectrometry, laser tomography and high-speed microphotography, and then tested its carbonation on genetically engineered mice.

To a vintner, the bubbly in a crystal flute may be vintage Dom Perignon or Pol Roger. But to these scientists, it is a complex hydroalcoholic solution supersaturated with carbon dioxide molecules and laced with proteins, lipids and amino acids.

Their experiments, described recently in Science, the American Scientist and the Proceedings of the National Academy of Sciences, reveal the unexpected ways in which the microscopic and molecular behavior of carbonation make champagne tingle in the nose and tap-dance on the tongue. Champagne owes much of its magic -- its savor, scent and glow -- to the micro-mechanics of CO2 bubbles, they reported.

"I could not imagine such beautiful hydrodynamic phenomena hidden right under our noses," says Gerard Liger-Belair, professor at the laboratory of enology and applied chemistry at the University of Reims Champagne-Ardenne in France, who has devoted a decade to deconstructing the fluid mechanics of sparkling wine.

Every bottle of champagne is a blend of many wines, but it owes its signature sparkle entirely to pent-up carbon dioxide. In fact, an average bottle of champagne contains about five or six times its volume in carbon dioxide, so compressed that when the champagne cork pops, it typically kicks out of the bottle's neck at about 30 miles per hour, Dr. Liger-Belair says. The champagne will actually taste better, he says, if the cork can be released with a more subdued CO2 sigh.

All this gas is the natural product of fermentation, created as yeast transforms sugar into ethanol. For centuries, though, winemakers treated so much CO2 as a hazard that, unvented, could make their wine bottles explode. The original developer of champagne is lost to legend, but an innovative 17th century Benedictine monk named Dom Pierre Perignon is sometimes credited with pioneering the predecessor of the wire collar, called a muselet, which holds a cork in place to withstand the fermentation pressure. With that simple twist, he turned a gassy nuisance into a luxury industry that last year sold about 322 million bottles of champagne world-wide.

For centuries, champagne makers considered pent-up carbon dioxide a hazard that could make their bottles explode. But the bubbles are so pleasing to the palette, it's no wonder 322 million bottles of champagne were sold world-wide last year. WSJ's Robert Lee Hotz reports.

Today, champagne makers ensure their lucrative bubbly is sufficiently saturated with CO2 by subjecting a base wine to a second round of fermentation inside tightly sealed bottles. After that, the CO2 pressure in the bottle is about six times the normal atmospheric pressure. When the bottle is uncorked, most of that gas quickly dissipates in a distinctive mist around the open bottle neck, but enough remains in the liquid to sire millions of bubbles, Dr. Liger-Belair says.

Traditionally, champagne bubbles were prized for their size and an aesthetic appeal that 19th century poet Lord Byron praised as "foaming whirls, as white as Cleopatra's pearls." Substituting the tools of chemistry for a wine-taster's more subjective judgments, Dr. Liger-Belair and his colleagues documented the expanding bubble universe within a glass of champagne.

In their findings, a bubble's biography begins inside a microscopic cellulose fiber clinging to the glass surface, usually fallen from the air or left by a towel. Gas builds up in the fiber as champagne splashes into the glass. When the combination of pressure, surface tension and viscosity is just right, the fiber starts leaking bubbles, the researchers said. Once settled after pouring, a glass of highly carbonated champagne effervesces at the rate of about 400 bubbles per second, compared with a rate of about 150 bubbles per second for beer, they reported. Champagne bubbles also are more flexible than beer bubbles, which affects how long they linger at the surface before popping.

As they rise, the bubbles swell to slightly less than a millimeter or so in diameter, absorbing other chemicals from the champagne. At the surface, they burst in a piquant froth.

Each exploding bubble sprays hundreds of droplets of concentrated compounds into the air, wreathing anyone drinking it in a fragrant mist, mass spectroscopy studies show. "These tiny droplets are highly concentrated, and this makes you feel directly through your nostrils all those aromatic molecules," Dr. Liger-Belair says.

Researchers used fluorescent dyes and laser imaging to monitor flow patterns. They discovered that the shape of a champagne glass can affect how thoroughly bubbles mix the beverage, which could affect its scent and flavor. Bubbles appeared to mix champagne more completely in a narrow, engraved flute than in the broad, shallow glass called a coupe.

Fizz, they found, seems to please the palette. Carbonated bubbles in sparkling wine, beer or soda actually activate our taste buds, researchers at Columbia University and University of California, San Diego recently reported.

Their discovery was inspired by reports of mountaineers who had lost their taste for bubbles after taking a medication called acetazolamide, which is used to prevent altitude sickness. After reaching the mountaintop, the climbers found that their beer tasted flat and soda tasted like dishwater. They dubbed the effect the champagne blues.

In October, biochemists Charles Zuker at Columbia and Jayaram Chandrashekar at UC San Diego showed that carbonation triggers an enzyme in taste buds that normally sense sourness. First, they implanted electrodes in normal mice to monitor a nerve connecting taste cells on the tongue. The nerve reacted to a taste of club soda or even a squirt of CO2 gas. The researchers then bred genetically engineered mice lacking those taste receptors and repeated the CO2 tests. "We can make a mouse with all its sour cells gone," Dr. Zuker says. "And when we make such a mouse, all CO2 sensing is gone."

Eventually, they identified a single sour-cell gene, called Car4, responsible for the enzyme sensitive to the taste of CO2. They found that the Car4 enzyme is also blocked by the altitude medication. "The zing and the tingle you get on your tongue is the stimulation of the sour receptors," says Dr. Zuker.

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